Cardiac prostheses for total cardiac replacement are, of course, known, and cases of total cardiac replacement using a cardiac prosthesis are attended by a great deal of publicity. Thanks to recent advances in technology and medical procedures, it is now possible to treat critically ill heart patients in some cases by total replacement of a defective heart with a cardiac prosthesis. The development and refinement of cardiac prosthesis has been the subject of intensive research in recent years, and great advances in this area are being made.
One of the more promising types of cardiac prosthesis which has been developed is a two-chamber device made of a semi-rigid urethane polymer. This device has a fluid chamber and a blood chamber separated by a flexible diaphragm. The blood chamber contains prosthetic heart valves to regulate the entry and exit of blood to and from the blood chamber. The pumping action of the prosthesis is controlled by a "heart driver" whose function is to impart a pumping action to the flexible diaphragm. This may be done mechanically, hydraulically or pneumatically. For example, in one type of pneumatic unit the heart driver is connected to the fluid chamber of the cardiac prosthesis by a flexible plastic tube connected to a source of compressed air. The heart driver controls the supply of air so as to alternately supply compressed air to the fluid chamber during systole (i.e., during the phase of the heart's operation where blood is pumped from the heart) and then to exhaust or draw air from the fluid chamber during diastole (i.e., that phase of the heart's operation where the heart is relaxed and the blood re-enters the heart chamber). During systole, the compressed air supplied to the fluid chamber exerts a force on the blood contained in the blood chamber via the flexible diaphragm. This force causes the flexible diaphragm to expel the blood from the blood chamber through the outflow valve. The duration of the systolic phase of operation is controlled by the heart driver. At the end of the systolic phase, the air is exhausted or drawn from the fluid chamber. This releases the pressure on the flexible diaphragm so that blood can refill the blood chamber via the inflow valve.
One lingering problem that has plagued the use of cardiac prostheses is the difficulty of obtaining reliable direct measurements of hemodynamic parameters such as blood pressure in the blood chamber. Although several methods have been employed to measure blood pressure, all of them have severe practical problems or risks associated with their use in that they invariably require some type of invasive technique.
For example, blood pressure may be measured using a catheter tipped transducer (e.g., the well-known Millar catheter) which must be inserted through the wall of the blood chamber through a special port built into the chamber wall to place the catheter. This port introduces a discontinuity in the flow of blood through the prosthesis and may eventually become a site of thrombogenesis, or clotting. In some cases a clot so formed can move through the patient's blood stream and lodge in the brain, causing a stroke. In addition, a typical Millar catheter can cost several thousand dollars, and is highly fragile and irreparable.
Because of the difficulty in using invasive techniques for measuring blood pressure, and the potential for thrombogenesis, these techniques are not practical for research or clinical cardiac prostheses use.